Multiple origins of cholinergic innervation of the cochlear nucleus

Cholinergic modulation in the vertebrate auditory pathway

Acetylcholine (ACh) is a prevalent neurotransmitter throughout the nervous system. In the brain, ACh is widely regarded as a potent neuromodulator. In neurons, ACh signals are conferred through a variety of receptors that influence a broad range of neurophysiological phenomena such as transmitter release or membrane excitability. In sensory circuitry, ACh modifies neural responses to stimuli and coordinates the activity of neurons across multiple levels of processing. These factors enable individual neurons or entire circuits to rapidly adapt to the dynamics of complex sensory stimuli, underscoring an essential role for ACh in sensory processing. In the auditory system, histological evidence shows that acetylcholine receptors (AChRs) are expressed at virtually every level of the ascending auditory pathway. Despite its apparent ubiquity in auditory circuitry, investigation of the roles of this cholinergic network has been mainly focused on the inner ear or forebrain structures, while less attention has been directed at regions between the cochlear nuclei and midbrain. In this review, we highlight what is known about cholinergic function throughout the auditory system from the ear to the cortex, but with a particular emphasis on brainstem and midbrain auditory centers. We will focus on receptor expression, mechanisms of modulation, and the functional implications of ACh for sound processing, with the broad goal of providing an overview of a newly emerging view of impactful cholinergic modulation throughout the auditory pathway.

Characterization of three cholinergic inputs to the cochlear nucleus

Acetylcholine modulates responses throughout the auditory system, including at the earliest brain level, the cochlear nucleus (CN). Previous studies have shown multiple sources of cholinergic input to the CN but information about their relative contributions and the distribution of inputs from each source is lacking. Here, we used staining for cholinergic axons and boutons, retrograde tract tracing, and acetylcholine-selective anterograde tracing to characterize three sources of acetylcholine input to the CN in mice. Staining for cholinergic axons showed heavy cholinergic inputs to granule cell areas and the dorsal CN with lighter input to the ventral CN. Retrograde tract tracing revealed that cholinergic cells from the superior olivary complex, pontomesencephalic tegmentum, and lateral paragigantocellular nucleus send projections to the CN. When we selectively labeled cholinergic axons from each source to the CN, we found surprising similarities in their terminal distributions, with patterns that were overlapping rather than complementary. Each source heavily targeted granule cell areas and the dorsal CN (especially the deep dorsal CN) and sent light input into the ventral CN. Our results demonstrate convergence of cholinergic inputs from multiple sources in most regions of the CN and raise the possibility of convergence onto single CN cells. Linking sources of acetylcholine and their patterns of activity to modulation of specific cell types in the CN will be an important next step in understanding cholinergic modulation of early auditory processing.

Olivocochlear projections contribute to superior intensity coding in cochlear nucleus small cells

Understanding communication signals, especially in noisy environments, is crucial to social interactions. Yet, as we age, acoustic signals can be disrupted by cochlear damage and the subsequent auditory nerve fibre degeneration. The most vulnerable medium- and high-threshold-auditory nerve fibres innervate various cell types in the cochlear nucleus, among which the small cells are unique in receiving this input exclusively. Furthermore, small cells project to medial olivocochlear (MOC) neurons, which in turn send branched collaterals back into the small cell cap. Here, we use single-unit recordings to characterise small cell firing characteristics and demonstrate superior intensity coding in this cell class. We show converse effects when activating/blocking the MOC system, demonstrating that small-cell unique coding properties are facilitated by direct cholinergic input from the MOC system. Small cells also maintain tone-level coding in the presence of background noise. Finally, small cells precisely code low-frequency modulation more accurately than other ventral cochlear nucleus cell types, demonstrating accurate envelope coding that may be important for vocalisation processing. These results highlight the small cell olivocochlear circuit as a key player in signal processing in noisy environments, which may be selectively degraded in ageing or after noise insult. KEY POINTS: Cochlear nucleus small cells receive input from low/medium spontaneous rate auditory nerve fibres and medial olivocochlear neurons. Electrical stimulation of medial olivocochlear neurons in the ventral nucleus of the trapezoid body and blocking cholinergic input to small cells using atropine demonstrates an excitatory cholinergic input to small cells, which increases responses to suprathreshold sound. Unique inputs to small cells produce superior sound intensity coding. This coding of intensity is preserved in the presence of background noise, an effect exclusive to this cell type in the cochlear nucleus. These results suggest that small cells serve an essential function in the ascending auditory system, which may be relevant to disorders such as hidden hearing loss.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Enzymes of acetylcholine metabolism in the rat inferior colliculus

Although there is strong evidence for cholinergic projections to the rat inferior colliculus, especially from the pedunculopontine tegmental nucleus (Noftz et al., 2020), there is a lack of information about the quantitative prevalence of the enzymes of acetylcholine metabolism in its various portions. We have used microdissection of freeze-dried sections combined with radiometric assays to map the distributions in the rat inferior colliculus of the activities of choline acetyltransferase (ChAT), which catalyzes synthesis of acetylcholine, and acetylcholinesterase (AChE), which catalyzes its breakdown by hydrolysis. Both enzyme activities were present throughout the inferior colliculus. Average ChAT activity was consistently somewhat higher in the external cortex, excluding its most superficial layer, than in the dorsal cortex or central nucleus. Within the external cortex, ChAT activity was about half as high laterally in its most superficial layer as elsewhere. The distribution of AChE activity was more uniform than that of ChAT. Overall, ChAT activity in the rat inferior colliculus was relatively low, about a fifth of that in whole brain of rat and lower than in other central auditory regions, whereas AChE activity was about two-thirds that of rat whole brain and about average for central auditory regions. The results are compared to previous measurements for cat and hamster inferior colliculus. They are consistent with a modest role for cholinergic neurotransmission in the inferior colliculus, to modulate the activity of its major neuronal types.

Effects of brainstem lesions on amino acid levels in the rat cochlear nucleus

There is evidence for glutamate, γ-amino butyric acid (GABA), and glycine as neurotransmitters of centrifugal pathways to the cochlear nucleus, but the quantitative extent of their contributions to amino acid neurotransmission in cochlear nucleus regions has not been known. We used microdissection of freeze-dried tissue sections of rat cochlear nucleus, with mapping of sample locations, combined with a high performance liquid chromatography (HPLC) assay, to measure amino acid levels in cochlear nucleus subregions of rats with unilateral lesions of centrifugal pathways to the cochlear nucleus. In rats with lesions transecting all or almost all pathways to the cochlear nucleus from brain stem regions, GABA, aspartate, and glutamate levels were reduced, compared to contralateral values, in almost all ipsilateral cochlear nucleus regions. The largest reductions, in dorsal (DCN), anteroventral (AVCN), and posteroventral (PVCN) cochlear nucleus regions, approached 50% for GABA, 40% for aspartate, and 30% for glutamate. In contrast, glutamine and taurine levels were typically higher in lesioned-side cochlear nucleus regions than contralaterally. Effects on glycine levels were mixed but usually included increased lesioned-side values compared to contralateral, probably reflecting a balance between increases during protein breakdown and decreases of free glycine in transected pathways. More limited lesions transecting just dorsal pathways showed much less effect on amino acid levels. Lesion of the ipsilateral trapezoid body connection plus ipsilateral superior olivary nuclei resulted in decreases of GABA, aspartate, and glutamate levels especially in ventral cochlear nucleus regions. No clear contralateral effects of this lesion could be shown. The results most strongly support centrifugal GABAergic pathways to the cochlear nucleus, providing almost half of GABAergic neurotransmission in most regions. Our results support and extend previously published measurements of lesion effects on GABA uptake and release in cochlear nucleus subdivisions.

Endogenous Cholinergic Signaling Modulates Sound-Evoked Responses of the Medial Nucleus of the Trapezoid Body

The medial nucleus of trapezoid body (MNTB) is a major source of inhibition in auditory brainstem circuitry. The MNTB projects well-timed inhibitory output to principal sound-localization nuclei in the superior olive (SOC) as well as other computationally important centers. Acoustic information is conveyed to MNTB neurons through a single calyx of Held excitatory synapse arising from the cochlear nucleus. The encoding efficacy of this large synapse depends on its activity rate, which is primarily determined by sound intensity and stimulus frequency. However, MNTB activity rate is additionally influenced by inhibition and possibly neuromodulatory inputs, albeit their functional role is unclear. Happe and Morley (2004) discovered prominent expression of α7 nAChRs in rat SOC, suggesting possible engagement of ACh-mediated modulation of neural activity in the MNTB. However, the existence and nature of this putative modulation have never been physiologically demonstrated. We probed nicotinic cholinergic influences on acoustic responses of MNTB neurons from adult gerbils (Meriones unguiculatus) of either sex. We recorded tone-evoked MNTB single-neuron activity in vivo using extracellular single-unit recording. Piggyback multibarrel electrodes enabled pharmacological manipulation of nAChRs by reversibly applying antagonists to two receptor types, α7 and α4β2. We observed that tone-evoked responses are dependent on ACh modulation by both nAChR subtypes. Spontaneous activity was not affected by antagonist application. Functionally, we demonstrate that ACh contributes to sustaining high discharge rates and enhances signal encoding efficacy. Additionally, we report anatomic evidence revealing novel cholinergic projections to MNTB arising from pontine and superior olivary nuclei.SIGNIFICANCE STATEMENT This study is the first to physiologically probe how acetylcholine, a pervasive neuromodulator in the brain, influences the encoding of acoustic information by the medial nucleus of trapezoid body, the most prominent source of inhibition in brainstem sound-localization circuitry. We demonstrate that this cholinergic input enhances neural discrimination of tones from noise stimuli, which may contribute to processing important acoustic signals, such as speech. Additionally, we describe novel anatomic projections providing cholinergic input to the MNTB. Together, these findings shed new light on the contribution of neuromodulation to fundamental computational processes in auditory brainstem circuitry and to a more holistic understanding of modulatory influences in sensory processing.

Identification of an inhibitory neuron subtype, the L-stellate cell of the cochlear nucleus

Auditory processing depends upon inhibitory signaling by interneurons, even at its earliest stages in the ventral cochlear nucleus (VCN). Remarkably, to date only a single subtype of inhibitory neuron has been documented in the VCN, a projection neuron termed the D-stellate cell. With the use of a transgenic mouse line, optical clearing, and imaging techniques, combined with electrophysiological tools, we revealed a population of glycinergic cells in the VCN distinct from the D-stellate cell. These multipolar glycinergic cells were smaller in soma size and dendritic area, but over ten-fold more numerous than D-stellate cells. They were activated by auditory nerve and T-stellate cells, and made local inhibitory synaptic contacts on principal cells of the VCN. Given their abundance, combined with their narrow dendritic fields and axonal projections, it is likely that these neurons, here termed L-stellate cells, play a significant role in frequency-specific processing of acoustic signals.

Chemical Effects of Kainic Acid Injection into the Rat Superior Olivary Region

Kainic acid injections have been used to destroy neuron somata in particular regions without damaging fiber tracts. We injected a solution of kainic acid into the region of the rat superior olivary complex in an effort to destroy its cholinergic projections to the cochlea and cochlear nucleus, which derive especially from the lateral superior olivary nucleus and ventral nucleus of the trapezoid body. In the lateral superior olivary nucleus, there were relatively small but fairly consistent decreases of choline acetyltransferase (ChAT) activity, larger decreases of acetylcholinesterase (AChE) activity, and consistent decreases of malate dehydrogenase activity, as a marker for oxidative metabolism. Other superior olivary regions were less affected by the kainic acid injections, but most showed overall significant decreases of AChE activity. Our results suggest that the cholinergic neurons giving rise to the centrifugal pathways to the cochlea and cochlear nucleus are more resistant to the effects of kainic acid than are those that receive major ascending input from the cochlear nucleus and project to higher levels of the auditory system. Comparison with published anatomical studies suggests that this resistance to the effects of kainic acid is related to relatively little glutamatergic input to the somata and proximal dendrites of these neurons. We also found a consistent approximately 16 % decrease of ChAT activity in the injected-side facial nerve root, which is most easily explained as a small effect of kainic acid on the facial nerve fibers passing through the injection site.

Muscarinic modulation of M and h currents in gerbil spherical bushy cells

Descending cholinergic fibers innervate the cochlear nucleus. Spherical bushy cells, principal neurons of the anterior part of the ventral cochlear nucleus, are depolarized by cholinergic agonists on two different time scales. A fast and transient response is mediated by alpha-7 homomeric nicotinic receptors while a slow and long-lasting response is mediated by muscarinic receptors. Spherical bushy cells were shown to express M3 receptors, but the receptor subtypes involved in the slow muscarinic response were not physiologically identified yet. Whole-cell patch clamp recordings combined with pharmacology and immunohistochemistry were performed to identify the muscarinic receptor subtypes and the effector currents involved. Spherical bushy cells also expressed both M1 and M2 receptors. The M1 signal was stronger and mainly somatic while the M2 signal was localized in the neuropil and on the soma of bushy cells. Physiologically, the M-current was observed for the gerbil spherical bushy cells and was inhibited by oxotremorine-M application. Surprisingly, long application of carbachol showed only a transient depolarization. Even though no muscarinic depolarization could be detected, the input resistance increased suggesting a decrease in the cell conductance that matched with the closure of M-channels. The hyperpolarization-activated currents were also affected by muscarinic activation and counteracted the effect of the inactivation of M-current on the membrane potential. We hypothesize that this double muscarinic action might allow adaptation of effects during long durations of cholinergic activation.

Amino acid and acetylcholine chemistry in mountain beaver cochlear nucleus and comparisons to pocket gopher, other rodents, and cat

The mountain beaver and pocket gopher are two rodents that live mostly underground in tunnel systems. Previous studies have suggested that their cochlear nucleus structure, particularly that of the dorsal cochlear nucleus (DCN), differs significantly from that of other mammals, that the hearing ability of the pocket gopher is deficient compared to that of other rodents, and that the DCN of the mountain beaver is more responsive to slow oscillations of air pressure than to sounds. We conducted some electrophysiological recordings from mountain beaver DCN and then used microchemical methods to map in mountain beaver cochlear nuclei the distributions of amino acids, including the major neurotransmitters of the brain, and enzyme activities related to the metabolism of the neurotransmitter acetylcholine, which functions in centrifugal pathways to the cochlear nucleus. Similar measurements were made for a pocket gopher cochlear nucleus. Responses to tonal stimuli were found in mountain beaver DCN. Distributions and magnitudes of neurotransmitter and related amino acids within mountain beaver and pocket gopher cochlear nuclei were not very different from those of other rodents and cat. However, the enzyme of synthesis for acetylcholine, choline acetyltransferase, had only low activities in the DCN of both mountain beaver and pocket gopher. The chemical distributions in the mountain beaver DCN support a conclusion that it corresponds to just the superficial DCN portion of other mammals. High correlations between the concentrations of γ-aminobutyrate (GABA) and glycine were found for both mountain beaver and pocket gopher cochlear nuclei, suggesting that their co-localization in cochlear nucleus synapses may be especially prominent in these animals. Previous evidence suggests convergence of somatosensory and auditory information in the DCN, and this may be especially true in animals spending most of their time underground. Our results suggest that the enlarged DCN of the mountain beaver and that of the pocket gopher are not very different from those of other rodents with respect to involvement of amino acid neurotransmitters, but they appear to have reduced cholinergic innervation.

The Role of the Brainstem in Generating and Modulating Tinnitus

Purpose The purpose of this work is to present a perspective article summarizing ideas pertaining to the brainstem's role in generating and modulating tinnitus. It is organized in 4 sections: Part 1, the role of the brainstem as a tinnitus generator; Part 2, the role of the brainstem in modulating tinnitus; Part 3, the role of the brainstem in nonauditory comorbid conditions associated with tinnitus; and Part 4, clinical implications. In Part 1, well-established neurophysiological models are discussed providing the framework of evidence that auditory brainstem nuclei play a role in generating tinnitus. In Part 2, ideas are presented explaining modulatory effects on tinnitus related to underlying pathways originating from or projecting to brainstem auditory and nonauditory nuclei. This section addresses multiple phenomena including somatic-related, attention-mediated, and emotion-mediated changes in the tinnitus percept. In Part 3, the role of the brainstem in common nonauditory comorbidities that occur in patients with tinnitus is discussed. Part 4 presents clinical implications of these new ideas related to the brainstem's involvement in generating and modulating tinnitus. Impact Knowledge of the brainstem's involvement in generating and modulating tinnitus provides a context for health care professionals to understand the temporal relationship between tinnitus and common nonauditory comorbid conditions.

Quantitative distribution of choline acetyltransferase activity in rat trapezoid body

There is evidence for a function of acetylcholine in the cochlear nucleus, primarily in a feedback, modulatory effect on auditory processing. Using a microdissection and quantitative microassay approach, choline acetyltransferase activity was mapped in the trapezoid bodies of rats, in which the activity is relatively higher than in cats or hamsters. Maps of series of sections through the trapezoid body demonstrated generally higher choline acetyltransferase activity rostrally than caudally, particularly in its portion ventral to the medial part of the spinal trigeminal tract. In the lateral part of the trapezoid body, near the cochlear nucleus, activities tended to be higher in more superficial portions than in deeper portions. Calculation of choline acetyltransferase activity in the total trapezoid body cross-section of a rat with a comprehensive trapezoid body map gave a value 3-4 times that estimated for the centrifugal labyrinthine bundle, which is mostly composed of the olivocochlear bundle, in the same rat. Comparisons with other rats suggest that the ratio may not usually be this high, but it is still consistent with our previous results suggesting that the centrifugal cholinergic innervation of the rat cochlear nucleus reaching it via a trapezoid body route is much higher than that reaching it via branches from the olivocochlear bundle. The higher choline acetyltransferase activity rostrally than caudally in the trapezoid body is consistent with evidence that the centrifugal cholinergic innervation of the cochlear nucleus derives predominantly from locations at or rostral to its anterior part, in the superior olivary complex and pontomesencephalic tegmentum.

Cholinergic responses of acoustically-characterized cochlear nucleus neurons: An in vivo iontophoretic study in Guinea pig

The responses of guinea pig cochlear nucleus neurons to in vivo iontophoretic application of various neurotransmitter agonists were recorded with extracellular multi-barrelled electrodes. Where possible, neurons were physiologically identified using strict criteria. Emphasis was placed on the action of cholinergic agonists in relation to the possible action of olivocochlear collateral innervation. Excitatory responses (increase in action potential firing) to glutamate were confirmed in a number of neuronal response types. Application of acetylcholine (ACh) or the broad spectrum cholinergic agonist carbachol produced reliable excitatory responses in about 47% of neurons (n = 29 out of 61 neurons). The remaining neurons were unresponsive to cholinergic agonists and no inhibitory responses were observed. Cholinergic responses were more common in dorsal cochlear nucleus (DCN) (73% of 30 neurons tested) than in ventral cochlear nucleus (VCN) (23% of 31 neurons). Of the total neuron sample in which cholinergic responses were investigated, 41 neurons were able to be categorized according to established acoustic response features. Excitatory responses to cholinergic agonists were seen in "Pauser-buildup" (Pb) and "Transient chopper" (Ct) response types. Primary-like neurons (PL and Pn) as well as "Onset chopper" (Oc) neurons (n = 6) were unresponsive to either ACh or carbachol. Oc neurons also did not show any effect on their acoustic responses. Robust cholinergic responses were also seen in several VCN and DCN neurons that were either unresponsive to sound, or had acoustic response properties that did not fit standard classification. The results suggest a relatively more robust cholinergic innervation of DCN compared to VCN. The excitatory cholinergic responses of some Ct neurons and the lack of effect on Oc neurons are consistent with previous results in mouse brain slice studies, but are in conflict with reports of medial olivocochlear collateral excitatory responses in onset-type neurons in vivo. The results also indicate that a number of neurons of unknown identity may also receive cholinergic input.

Stereological Estimates of Glutamatergic, GABAergic, and Cholinergic Neurons in the Pedunculopontine and Laterodorsal Tegmental Nuclei in the Rat

The pedunculopontine tegmental nucleus (PPN) and laterodorsal tegmental nucleus (LDT) are functionally associated brainstem structures implicated in behavioral state control and sensorimotor integration. The PPN is also involved in gait and posture, while the LDT plays a role in reward. Both nuclei comprise characteristic cholinergic neurons intermingled with glutamatergic and GABAergic cells whose absolute numbers in the rat have been only partly established. Here we sought to determine the complete phenotypical profile of each nucleus to investigate potential differences between them. Counts were obtained using stereological methods after the simultaneous visualization of cholinergic and either glutamatergic or GABAergic cells. The two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67, were separately analyzed. Dual in situ hybridization revealed coexpression of GAD65 and GAD67 mRNAs in ∼90% of GAD-positive cells in both nuclei; thus, the estimated mean numbers of (1) cholinergic, (2) glutamatergic, and (3) GABAergic cells in PPN and LDT, respectively, were (1) 3,360 and 3,650; (2) 5,910 and 5,190; and (3) 4,439 and 7,599. These data reveal significant differences between PPN and LDT in their relative phenotypical composition, which may underlie some of the functional differences observed between them. The estimation of glutamatergic cells was significantly higher in the caudal PPN, supporting the reported functional rostrocaudal segregation in this nucleus. Finally, a small subset of cholinergic neurons (8% in PPN and 5% in LDT) also expressed the glutamatergic marker Vglut2, providing anatomical evidence for a potential corelease of transmitters at specific target areas.

Radiate and Planar Multipolar Neurons of the Mouse Anteroventral Cochlear Nucleus: Intrinsic Excitability and Characterization of their Auditory Nerve Input

Radiate and planar neurons are the two major types of multipolar neurons in the ventral cochlear nucleus (VCN). Both cell types receive monosynaptic excitatory synaptic inputs from the auditory nerve, but have different responses to sound and project to different target regions and cells. Although the intrinsic physiology and synaptic inputs to planar neurons have been previously characterized, the radiate neurons are less common and have not been as well studied. We studied both types of multipolar neurons and characterized their properties including intrinsic excitability, synaptic dynamics of their auditory nerve inputs, as well as their neural firing properties to auditory nerve stimulation. Radiate neurons had a faster member time constant and higher threshold current to fire spikes than planar neurons, but the maximal firing rate is the same for both cell types upon large current injections. Compared to planar neurons, radiate neurons showed spontaneous postsynaptic currents with smaller size, and slower but variable kinetics. Auditory nerve stimulation progressively recruited synaptic inputs that were smaller and slower in radiate neurons, over a broader range of stimulus strength. Synaptic inputs to radiate neurons showed less depression than planar neurons during low rates of repetitive activity, but the synaptic depression at higher rates was similar between two cell types. However, due to the slow kinetics of the synaptic inputs, synaptic transmission in radiate neurons showed prominent temporal summation that contributed to greater synaptic depolarization and a higher firing rate for repetitive auditory nerve stimulation at high rates. Taken together, these results show that radiate multipolar neurons integrate a large number of weak synaptic inputs over a broad dynamic range, and have intrinsic and synaptic properties that are distinct from planar multipolar neurons. These properties enable radiate neurons to generate powerful inhibitory inputs to target neurons during high levels of afferent activity. Such robust inhibition is expected to dynamically modulate the excitability of many cell types in the cochlear nuclear complex.

Small Compartment Toxicity: CN VIII and Quality of Life: Hearing Loss, Tinnitus, and Balance Disorders

Life experiences, industrial/environmental exposures, and administration of Food and Drug Administration (FDA)-approved drugs may have unintended but detrimental effects on peripheral and central auditory pathways. Most relevant to the readership of this journal is the role that drug treatments approved by the FDA as safe and effective appear to interact with 3 independent modes of toxicity within the small compartment of the ear. What may seem to be trivial drug-induced toxicity has the potential to change important measures of quality of life and functional capacity of mid- to late-life patients. Drugs meant to treat can become the source of interference in the activities of daily living, and as a result, treatment compliance may be jeopardized. Ototoxicity has been defined as the tendency of certain therapeutic agents and other chemical substances to cause functional impairments and cellular degeneration of the tissues of the inner ear resulting in hearing loss. However, one of the largest contributors to hospitalizations is fall-related injuries in the elderly patients associated with disorders of vestibular function linked to progressive and drug-induced toxicities. Tinnitus affects 35 to 50 million adults representing approximately 25% of the US population, with 12 million seeking medical care and 2 to 3 million reporting symptoms that were severely debilitating. This review is intended to highlight these targets of neurotoxicity that threaten the usefulness of drug treatments deemed safe and effective prior to access by the general public.

Cholinergic top-down influences on the auditory brainstem

Descending connections are present in many sensory systems and support adaptive information processing. This allows the sensory brain to code a wider range of inputs. A well characterized descending system is the olivo-cochlear cholinergic innervation of the inner ear, which mediates a reduction of the sensitivity of the inner ear upon perception of intense sounds. Because this inhibits the response to background noise, the olivo-cochlear system supports detection of transient sound events. Olivo-cochlear neurons also innervate the cochlear nucleus through axon collaterals. Here, acetylcholine increases the excitability of central neurons without reducing their temporal precision. Thus their target neurons in the superior olivary complex can more effectively process binaural temporal cues. We argue that the central effect of the olivo-cochlear system augments the peripheral effect. In addition, olivo-cochlear cholinergic neurons are under top-down control of cortical inputs, providing further adaptability of information processing on the level of the auditory brainstem.

Muscarinic acetylcholine receptors control baseline activity and Hebbian stimulus timing-dependent plasticity in fusiform cells of the dorsal cochlear nucleus

Cholinergic modulation contributes to adaptive sensory processing by controlling spontaneous and stimulus-evoked neural activity and long-term synaptic plasticity. In the dorsal cochlear nucleus (DCN), in vitro activation of muscarinic acetylcholine receptors (mAChRs) alters the spontaneous activity of DCN neurons and interacts with N-methyl-d-aspartate (NMDA) and endocannabinoid receptors to modulate the plasticity of parallel fiber synapses onto fusiform cells by converting Hebbian long-term potentiation to anti-Hebbian long-term depression. Because noise exposure and tinnitus are known to increase spontaneous activity in fusiform cells as well as alter stimulus timing-dependent plasticity (StTDP), it is important to understand the contribution of mAChRs to in vivo spontaneous activity and plasticity in fusiform cells. In the present study, we blocked mAChRs actions by infusing atropine, a mAChR antagonist, into the DCN fusiform cell layer in normal hearing guinea pigs. Atropine delivery leads to decreased spontaneous firing rates and increased synchronization of fusiform cell spiking activity. Consistent with StTDP alterations observed in tinnitus animals, atropine infusion induced a dominant pattern of inversion of StTDP mean population learning rule from a Hebbian to an anti-Hebbian profile. Units preserving their initial Hebbian learning rules shifted toward more excitatory changes in StTDP, whereas units with initial suppressive learning rules transitioned toward a Hebbian profile. Together, these results implicate muscarinic cholinergic modulation as a factor in controlling in vivo fusiform cell baseline activity and plasticity, suggesting a central role in the maladaptive plasticity associated with tinnitus pathology.NEW & NOTEWORTHY This study is the first to use a novel method of atropine infusion directly into the fusiform cell layer of the dorsal cochlear nucleus coupled with simultaneous recordings of neural activity to clarify the contribution of muscarinic acetylcholine receptors (mAChRs) to in vivo fusiform cell baseline activity and auditory-somatosensory plasticity. We have determined that blocking the mAChRs increases the synchronization of spiking activity across the fusiform cell population and induces a dominant pattern of inversion in their stimulus timing-dependent plasticity. These modifications are consistent with similar changes established in previous tinnitus studies, suggesting that mAChRs might have a critical contribution in mediating the maladaptive alterations associated with tinnitus pathology. Blocking mAChRs also resulted in decreased fusiform cell spontaneous firing rates, which is in contrast with their tinnitus hyperactivity, suggesting that changes in the interactions between the cholinergic and GABAergic systems might also be an underlying factor in tinnitus pathology.

Slow Cholinergic Modulation of Spike Probability in Ultra-Fast Time-Coding Sensory Neurons

Sensory processing in the lower auditory pathway is generally considered to be rigid and thus less subject to modulation than central processing. However, in addition to the powerful bottom-up excitation by auditory nerve fibers, the ventral cochlear nucleus also receives efferent cholinergic innervation from both auditory and nonauditory top-down sources. We thus tested the influence of cholinergic modulation on highly precise time-coding neurons in the cochlear nucleus of the Mongolian gerbil. By combining electrophysiological recordings with pharmacological application in vitro and in vivo, we found 55-72% of spherical bushy cells (SBCs) to be depolarized by carbachol on two time scales, ranging from hundreds of milliseconds to minutes. These effects were mediated by nicotinic and muscarinic acetylcholine receptors, respectively. Pharmacological block of muscarinic receptors hyperpolarized the resting membrane potential, suggesting a novel mechanism of setting the resting membrane potential for SBC. The cholinergic depolarization led to an increase of spike probability in SBCs without compromising the temporal precision of the SBC output in vitro. In vivo, iontophoretic application of carbachol resulted in an increase in spontaneous SBC activity. The inclusion of cholinergic modulation in an SBC model predicted an expansion of the dynamic range of sound responses and increased temporal acuity. Our results thus suggest of a top-down modulatory system mediated by acetylcholine which influences temporally precise information processing in the lower auditory pathway.

Cholinergic modulation of auditory steady-state response in the auditory cortex of the freely moving rat

As disturbance in auditory steady-state response (ASSR) has been consistently found in many neuropsychiatric disorders, such as autism spectrum disorder and schizophrenia, there is considerable interest in the development of translational rat models to elucidate the underlying neural and neurochemical mechanisms involved in ASSR. This is the first study to investigate the effects of the non-selective muscarinic antagonist scopolamine and the cholinesterase inhibitor donepezil (also in combination with scopolamine) on ASSR. We recorded the local field potentials through the chronic microelectrodes implanted in the auditory cortex of freely moving rat. ASSRs were recorded in response to auditory stimuli delivered over a range of frequencies (10-80 Hz) and averaged over 60 trials. We found that a single dose of scopolamine produced a temporal attenuation in response to auditory stimuli; the most attenuation occurred at 40 Hz. Time-frequency analysis revealed deficits in both power and phase-locking to 40 Hz. Donepezil augmented 40-Hz steady-state power and phase-locking. Scopolamine combined with donepezil had an enhanced effect on the phase-locking, but not power of ASSR. These changes induced by cholinergic drugs suggest an involvement of muscarinic neurotransmission in auditory processing and provide a rodent model investigating the neurochemical mechanism of neurophysiological deficits seen in patients.

Modulating central gain in tinnitus: changes in nitric oxide synthase in the ventral cochlear nucleus

A significant challenge in tinnitus research lies in explaining how acoustic insult leads to tinnitus in some individuals, but not others. One possibility is genetic variability in the expression and function of neuromodulators – components of neural signaling that alter the balance of excitation and inhibition in neural circuits. An example is nitric oxide (NO) – a free radical and potent neuromodulator in the mammalian brain – that regulates plasticity via both pre-synaptic and postsynaptic mechanisms. Changes in NO have previously been implicated in tinnitus generation, specifically in the ventral cochlear nucleus (VCN). Here, we examined nitric oxide synthase (NOS) – the enzyme responsible for NO production – in the guinea pig VCN following acoustic trauma. NOS was present in most cell types – including spherical and globular bushy cells, small, medium, and large multipolar cells, and octopus cells – spanning the entire extent of the VCN. The staining pattern was symmetrical in control animals. Unilateral acoustic over-exposure (AOE) resulted in marked asymmetries between ipsilateral and contralateral sides of the VCN in terms of the distribution of NOS across the cochlear nuclei in animals with behavioral evidence of tinnitus: fewer NOS-positive cells and a reduced level of NOS staining was present across the whole extent of the contralateral VCN, relative to the ipsilateral VCN. The asymmetric pattern of NOS-containing cells was observed as early as 1 day after AOE and was also present in some animals at 3, 7, and 21 days after AOE. However, it was not until 8 weeks after AOE, when tinnitus had developed, that asymmetries were significant overall, compared with control animals. Asymmetrical NOS expression was not correlated with shifts in the threshold hearing levels. Variability in NOS expression between animals may represent one underlying difference that can be linked to whether or not tinnitus develops after noise exposure.

Increased contralateral suppression of otoacoustic emissions indicates a hyperresponsive medial olivocochlear system in humans with tinnitus and hyperacusis

Atypical medial olivocochlear (MOC) feedback from brain stem to cochlea has been proposed to play a role in tinnitus, but even well-constructed tests of this idea have yielded inconsistent results. In the present study, it was hypothesized that low sound tolerance (mild to moderate hyperacusis), which can accompany tinnitus or occur on its own, might contribute to the inconsistency. Sound-level tolerance (SLT) was assessed in subjects (all men) with clinically normal or near-normal thresholds to form threshold-, age-, and sex-matched groups: 1) no tinnitus/high SLT, 2) no tinnitus/low SLT, 3) tinnitus/high SLT, and 4) tinnitus/low SLT. MOC function was measured from the ear canal as the change in magnitude of distortion-product otoacoustic emissions (DPOAE) elicited by broadband noise presented to the contralateral ear. The noise reduced DPOAE magnitude in all groups ("contralateral suppression"), but significantly more reduction occurred in groups with tinnitus and/or low SLT, indicating hyperresponsiveness of the MOC system compared with the group with no tinnitus/high SLT. The results suggest hyperresponsiveness of the interneurons of the MOC system residing in the cochlear nucleus and/or MOC neurons themselves. The present data, combined with previous human and animal data, indicate that neural pathways involving every major division of the cochlear nucleus manifest hyperactivity and/or hyperresponsiveness in tinnitus and/or low SLT. The overactivation may develop in each pathway separately. However, a more parsimonious hypothesis is that top-down neuromodulation is the driving force behind ubiquitous overactivation of the auditory brain stem and may correspond to attentional spotlighting on the auditory domain in tinnitus and hyperacusis.

Cochlear damage affects neurotransmitter chemistry in the central auditory system

Tinnitus, the perception of a monotonous sound not actually present in the environment, affects nearly 20% of the population of the United States. Although there has been great progress in tinnitus research over the past 25 years, the neurochemical basis of tinnitus is still poorly understood. We review current research about the effects of various types of cochlear damage on the neurotransmitter chemistry in the central auditory system and document evidence that different changes in this chemistry can underlie similar behaviorally measured tinnitus symptoms. Most available data have been obtained from rodents following cochlear damage produced by cochlear ablation, intense sound, or ototoxic drugs. Effects on neurotransmitter systems have been measured as changes in neurotransmitter level, synthesis, release, uptake, and receptors. In this review, magnitudes of changes are presented for neurotransmitter-related amino acids, acetylcholine, and serotonin. A variety of effects have been found in these studies that may be related to animal model, survival time, type and/or magnitude of cochlear damage, or methodology. The overall impression from the evidence presented is that any imbalance of neurotransmitter-related chemistry could disrupt auditory processing in such a way as to produce tinnitus.

Cholinergic modulation of large-conductance calcium-activated potassium channels regulates synaptic strength and spine calcium in cartwheel cells of the dorsal cochlear nucleus

Acetylcholine is a neuromodulatory transmitter that controls synaptic plasticity and sensory processing in many brain regions. The dorsal cochlear nucleus (DCN) is an auditory brainstem nucleus that integrates auditory signals from the cochlea with multisensory inputs from several brainstem nuclei and receives prominent cholinergic projections. In the auditory periphery, cholinergic modulation serves a neuroprotective function, reducing cochlear output under high sound levels. However, the role of cholinergic signaling in the DCN is less understood. Here we examine postsynaptic mechanisms of cholinergic modulation at glutamatergic synapses formed by parallel fiber axons onto cartwheel cells (CWCs) in the apical DCN circuit from mouse brainstem slice using calcium (Ca) imaging combined with two-photon laser glutamate uncaging onto CWC spines. Activation of muscarinic acetylcholine receptors (mAChRs) significantly increased the amplitude of both uncaging-evoked EPSPs (uEPSPs) and spine Ca transients. Our results demonstrate that mAChRs in CWC spines act by suppressing large-conductance calcium-activated potassium (BK) channels, and this effect is mediated through the cAMP/protein kinase A signaling pathway. Blocking BK channels relieves voltage-dependent magnesium block of NMDA receptors, thereby enhancing uEPSPs and spine Ca transients. Finally, we demonstrate that mAChR activation inhibits L-type Ca channels and thus may contribute to the suppression of BK channels by mAChRs. In summary, we demonstrate a novel role for BK channels in regulating glutamatergic transmission and show that this mechanism is under modulatory control of mAChRs.

Morphological characterization of bushy cells and their inputs in the laboratory mouse (Mus musculus) anteroventral cochlear nucleus

Spherical and globular bushy cells of the AVCN receive huge auditory nerve endings specialized for high fidelity neural transmission in response to acoustic events. Recent studies in mice and other rodent species suggest that the distinction between bushy cell subtypes is not always straightforward. We conducted a systematic investigation of mouse bushy cells along the rostral-caudal axis in an effort to understand the morphological variation that gives rise to reported response properties in mice. We combined quantitative light and electron microscopy to investigate variations in cell morphology, immunostaining, and the distribution of primary and non-primary synaptic inputs along the rostral-caudal axis. Overall, large regional differences in bushy cell characteristics were not found; however, rostral bushy cells received a different complement of axosomatic input compared to caudal bushy cells. The percentage of primary auditory nerve terminals was larger in caudal AVCN, whereas non-primary excitatory and inhibitory inputs were more common in rostral AVCN. Other ultrastructural characteristics of primary auditory nerve inputs were similar across the rostral and caudal AVCN. Cross sectional area, postsynaptic density length and curvature, and mitochondrial volume fraction were similar for axosomatic auditory nerve terminals, although rostral auditory nerve terminals contained a greater concentration of synaptic vesicles near the postsynaptic densities. These data demonstrate regional differences in synaptic organization of inputs to mouse bushy cells rather than the morphological characteristic of the cells themselves.

Choline acetyltransferase activity in the hamster central auditory system and long-term effects of intense tone exposure

Acoustic trauma often leads to loss of hearing of environmental sounds, tinnitus, in which a monotonous sound not actually present is heard, and/or hyperacusis, in which there is an abnormal sensitivity to sound. Research on hamsters has documented physiological effects of exposure to intense tones, including increased spontaneous neural activity in the dorsal cochlear nucleus. Such physiological changes should be accompanied by chemical changes, and those chemical changes associated with chronic effects should be present at long times after the intense sound exposure. Using a microdissection mapping procedure combined with a radiometric microassay, we have measured activities of choline acetyltransferase (ChAT), the enzyme responsible for synthesis of the neurotransmitter acetylcholine, in the cochlear nucleus, superior olive, inferior colliculus, and auditory cortex of hamsters 5 months after exposure to an intense tone compared with control hamsters of the same age. In control hamsters, ChAT activities in auditory regions were never more than one-tenth of the ChAT activity in the facial nerve root, a bundle of myelinated cholinergic axons, in agreement with a modulatory rather than a dominant role of acetylcholine in hearing. Within auditory regions, relatively higher activities were found in granular regions of the cochlear nucleus, dorsal parts of the superior olive, and auditory cortex. In intense-tone-exposed hamsters, ChAT activities were significantly increased in the anteroventral cochlear nucleus granular region and the lateral superior olivary nucleus. This is consistent with some chronic upregulation of the cholinergic olivocochlear system influence on the cochlear nucleus after acoustic trauma.

Suppression of noise-induced hyperactivity in the dorsal cochlear nucleus following application of the cholinergic agonist, carbachol

Increased spontaneous firing (hyperactivity) is induced in fusiform cells of the dorsal cochlear nucleus (DCN) following intense sound exposure and is implicated as a possible neural correlate of noise-induced tinnitus. Previous studies have shown that in normal hearing animals, fusiform cell activity can be modulated by activation of parallel fibers, which represent the axons of granule cells. The modulation consists of a transient excitation followed by a more prolonged period of inhibition, presumably reflecting direct excitatory inputs to fusiform cells and an indirect inhibitory input to fusiform cells from the granule cell-cartwheel cell system. We hypothesized that since granule cells can be activated by cholinergic inputs, it might be possible to suppress tinnitus-related hyperactivity of fusiform cells using the cholinergic agonist, carbachol. To test this hypothesis, we recorded multiunit spontaneous activity in the fusiform soma layer (FSL) of the DCN in control and tone-exposed hamsters (10 kHz, 115 dB SPL, 4h) before and after application of carbachol to the DCN surface. In both exposed and control animals, 100 μM carbachol had a transient excitatory effect on spontaneous activity followed by a rapid weakening of activity to near or below normal levels. In exposed animals, the weakening of activity was powerful enough to completely abolish the hyperactivity induced by intense sound exposure. This suppressive effect was partially reversed by application of atropine and was usually not associated with significant changes in neural best frequencies (BF) or BF thresholds. These findings demonstrate that noise-induced hyperactivity can be pharmacologically controlled and raise the possibility that attenuation of tinnitus may be achievable by using an agonist of the cholinergic system.

Stimulus-timing dependent multisensory plasticity in the guinea pig dorsal cochlear nucleus

Multisensory neurons in the dorsal cochlear nucleus (DCN) show long-lasting enhancement or suppression of sound-evoked responses when stimulated with combined somatosensory-auditory stimulation. By varying the intervals between sound and somatosensory stimuli we show for the first time in vivo that DCN bimodal responses are influenced by stimulus-timing dependent plasticity. The timing rules and time courses of the observed stimulus-timing dependent plasticity closely mimic those of spike-timing dependent plasticity that have been demonstrated in vitro at parallel-fiber synapses onto DCN principal cells. Furthermore, the degree of inhibition in a neuron influences whether that neuron has Hebbian or anti-Hebbian timing rules. As demonstrated in other cerebellar-like circuits, anti-Hebbian timing rules reflect adaptive filtering, which in the DCN would result in suppression of sound-evoked responses that are predicted by activation of somatosensory inputs, leading to the suppression of body-generated signals such as self-vocalization.

Activation of muscarinic receptors increases the activity of the granule neurones of the rat dorsal cochlear nucleus--a calcium imaging study

Acetylcholine modulates the function of the cochlear nucleus via several pathways. In this study, the effects of cholinergic stimulation were studied on the cytoplasmic Ca(2+) concentration of granule neurones of the rat dorsal cochlear nucleus (DCN). Ca(2+) transients were recorded in Oregon-Green-BAPTA 1-loaded brain slices using a calcium imaging technique. For the detection, identification and characterisation of the Ca(2+) transients, a wavelet analysis-based method was developed. Granule cells were identified on the basis of their size and localisation. The action potential-coupled character of the Ca(2+) transients of the granule cells was established by recording fluorescence changes and electrical activity simultaneously. Application of the cholinergic agonist carbamyl-choline (CCh) significantly increased the frequency of the Ca(2+) transients (from 0.37 to 6.31 min(-1), corresponding to a 17.1-fold increase; n = 89). This effect was antagonised by atropine, whereas CCh could still evoke an 8.3-fold increase of the frequency of the Ca(2+) transients when hexamethonium was present. Using immunolabelling, the expression of both type 1 and type 3 muscarinic receptors (M1 and M3 receptors, respectively) was demonstrated in the granule cells. Application of 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (an M3-specific antagonist) prevented the onset of the CCh effect, whereas an M1-specific antagonist (pirenzepine) was less effective. We conclude that cholinergic stimulation increases the activity of granule cells, mainly by acting on their M3 receptors. The modulation of the firing activity of the granule cells, in turn, may modify the firing of projection neurones and may adjust signal processing in the entire DCN.

Patterns of olivocochlear axonal branches

The olivocochlear (OC) pathway is the source of major feedback control of ascending acoustic information. Two main patterns of axonal branching are evident at this and other levels of the auditory pathway: long-distance collaterals and branches involved in feedback-control loops. Only a minority of OC neurons project to both cochlea, consistent with a role for the system in sound localization. OC branches to the ventral cochlear nucleus provide the anatomical substrate for a feedback control loop that could aid in modulating the intensity of acoustic information being conveyed to higher cortical levels while still dampening the overall intensity of signaling from the cochlea itself.